1. Field of the Invention
The present invention relates to a communication apparatus which is connected to a communication network such as a public telephone line, or with which communication is carried out through the communication network, and a communication method therefor.
2. Description of the Related Art
Conventionally, communication terminal units such as various telephone sets can be connected to a public switched telephone network (hereinafter referred to as a "PSTN") using a public telephone line. For example, in a cordless telephone set, a master unit is connected to the PSTN in a wired state, and a plurality of slave units are connected to the master unit in a wireless state. Even such a cordless telephone set having a plurality of terminal units is handled as one terminal unit for the PSTN. For example, when a terminal unit connected to the PSTN serves as an originating side and calls a cordless telephone set, the originating side performs calls the cordless telephone set of the called side by using a telephone number corresponding to the master unit of the cordless telephone set of the called side. An exchange in the PSTN calls the master unit of the cordless telephone set corresponding to the telephone number sent from the terminal unit of the originating side. The master unit of the cordless telephone set called from the exchange of the PSTN connects all the slave units in a wireless state, and the master unit and all the slave units output termination ringings.
When terminal units have message exchange functions, assume that a message is transmitted from an originating terminal unit to a called terminal unit. In this case, when a telephone number corresponding to the called terminal unit is originated from the originating terminal unit, an exchange of the PSTN calls a terminal unit corresponding to the received telephone number. The called terminal unit outputs termination ringings and waits for a response executed by a user of the called terminal unit. At this time, assume that the called terminal unit is an automatic answering telephone set and set to execute an automatic recording function. In this case, after a predetermined number of termination ringings are made, an automatic answering operation is started to record a message transmitted from the originating terminal unit. A prior art related to an automatic answering telephone set is disclosed, for example, in Japanese Unexamined Patent Publications JP-A 3-112246 (1991), JP-A 4-14353 (1992).
When a plurality of terminal units including a master unit are connected to the PSTN, there is a demand for a function that only the master unit or one slave unit designated of a cordless telephone is called. For example, when slave units of a home cordless telephone set are individually assigned to a plurality of members of a family, it may be of occurrence that calling all the slave units is not necessarily required, but calling only one slave unit designated is desired. However, in the present PSTN, there is no protocol for designating inter-calling termination, and a terminal unit has no means for identifying inter-calling termination information from the communication network. For this reason, even when one slave unit of the cordless telephone set must be called, ringing of the master unit and all the slave units is performed.
In addition, in the PSTN, there is no protocol for designating message communication, and a terminal unit has no function of identifying message information from the communication network. For this reason, the message cannot be received to be processed in a normal termination condition.
Further, since a terminal unit connected to the PSTN has no function of identifying a carrier, a type 1 carrier participating in the PSTN cannot provide an unique information service and must be standardized to start a new service.